Corrosion of the metal reinforcement in concrete structures presents a serious threat to durability. It is not the deterioration of the concrete itself, but the corrosion of reinforcement that represents the greatest threat to the durability of concrete structures. Cracking and spalling of concrete under pressures created by the formation of voluminous corrosion products is a common occurence. By determining the extent of corrosion activity, life estimates can be made or preventative measures taken.
Presently, potential measurement methods require electrical connection to the reinforcement, and this only indicates that corrosion has taken place, it does not indicate the level of corrosion activity. The standard method of field investigating of corrosion behavior employs "half-cells" for determining the potential of the reinforcement at selected locations. The basic test configuration is shown in FIG. 1, where a high impedance voltmeter is employed to determine potentials relative to a copper-copper sulfate (Cu/CuSO.sub.4) reference electrode. Potential readings at the area of measurement are considered to indicate the following activity:
______________________________________ E.sub.corr &gt; -.20 volts 90% probality no corrosion -.20 &gt; E.sub.corr &gt; -.35 volts Corrosion uncertain E.sub.corr &lt; -.35 volt 90% possibility corrosion occurring ______________________________________
This method of potential measurement can result in useful data for structures such as bridge decks, particularly with the construction of equipotential contour maps. However, it has undeniable limitations. First, electrical connection with the reinforcement is required. Connection in an existing structure requires removal of concrete at numerous sites to insure electrical continuity to all areas being surveyed. This may be impractical for most inspections. Additionally, in highly conductive marine environments, concrete affects readings in a manner similar to nonconductive coatings; greatly increasing in the area associated with a single measurement (decreasing measurement localization). There is also the questionable range of potential between -0.20 and -0.35 volts (Cu/CuSO.sub.4) associated with the standard test method.
Investigation of pitting behavior with instrumented steel samples embedded in concrete clearly showed a drop in potential associated with the onset of pitting. During an induction period where no current flow is observed, a slow decrease in potential occurs. At approximately -0.25 volt (saturated calomel electrode [SCE]) there is a marked rise in current accompanied by a sharp drop in potential. This potential, -0.32 volt (Cu/CuSO.sub.4), is close to the -0.20 volt (Cu/SO.sub.4) specified as indicative of no corrosion activity. However, below -0.50 volt SCE (-0.57 volt Cu/CuSO.sub.4) current tapers off with a continued decrease in potential. Therefore, potential measurements in themselves may serve more as an indicator of corrosion history (that corrosion has been initiated ) than giving the level of corrosion activity.